Recent studies of noise-induced and age-related hearing loss show that loss of synapses between cochlear nerve terminals and inner hair cells (IHCs), rather than hair cell damage, is often the first degenerative event. Noise exposures causing only temporary threshold shifts, and no loss of hair cells, nevertheless cause rapid and permanent loss of IHC synapses, followed slowly by death of spiral ganglion neurons (SGNs). A similar view of the importance of synaptopathy has emerged with respect to the aging ear. Even without purposeful noise exposure, loss of IHC synapses in mice progresses steadily throughout life, long before the loss of hair cells and SGNs, and similar findings are emerging from post-mortem studies of human ears. Thus, understanding the mechanisms that underlie the formation, maintenance, and regeneration of the IHC-SGN synapse are key to understanding the cellular and molecular basis of acquired hearing loss and thus in the development of rational therapies. Using cell-specific, inducible gene recombination in several novel mouse transgenic lines, we showed that neurotrophin 3 (NT-3) derived from cochlear supporting cells is a key regulator of IHC synapse formation and maintenance in the neonate, and that neonatal NT-3 overexpression in supporting cells enhances synaptic and functional recovery after acoustic overexposure in the adult. Here we will test the hypotheses that 1) NT-3 is critical for regulation of IHC synapses in the adult, and 2) that age- related or noise-induced cochlear neuropathy can be modulated by up- or down-regulating NT-3. Aim 1 will determine the roles of NT-3 in regulating IHC synapses in the adult and aging ear by genetically inducing supporting-cell NT-3 overexpression or deletion in the adult cochlea and assessing the effects on cochlear structure and function over short (wks) and long (months) survival. Aim 2 will test the hypothesis that increases or decreases in cochlear NT-3 can influence the severity of, or recovery from, noise-induced cochlear neuropathy using genetic and pharmacological approaches. We will over- or under-express NT-3 in supporting cells by genetic means either 1 wk before or 1 day after a neuropathic noise exposure and monitor cochlear function via ABRs and DPOAEs at different intervals, out to 6 months. For the pharmacological approach, NT-3 will be delivered via round window application in a slow-release gel. For both aims, we will monitor changes in cochlear function via ABRs and DPOAEs, and cochleas will be collected either for RT-PCR analysis of NT-3 expression and/or for histological analysis of hair cell and SGN counts as well as IHC synapse number and morphology.